Several non-volatile memory technologies are currently being developed with various degrees of maturity. PCRAM (Phase Change RAM), CBRAM (Conductive Bridge RAM) or OxRAM (Oxide based RAM), FeRAM (Ferroelectric RAM) and MRAM (Magnetic RAM) may especially be cited. Apart from FeRAM memories which operate according to the principle of the orientation of an electric dipolar moment in a ferroelectric material, all the other memories use materials of variable electrical resistance. Each information bit is stored in a memory point including a variable resistance element. The information bit is encoded by the resistance value of this storage element. Typically, the logical level ‘0’ corresponds to a high resistance value and the logical level ‘1’ corresponds to a low resistance value.
The mechanism behind the variation in resistance depends on the technology used. In PCRAM for example, chalcogenide semi-conductor materials are used which can be made to transit from an amorphous state to a crystalline state (or vice-versa), by current pulses of suitable amplitude and duration. In MRAM, the storage element is a magnetic tunnel junction having a tunnel magnetoresistance effect. In CBRAM, conductive filaments are formed or destroyed by making metal ions (for example Ag+) diffuse in a semi-conducting array (for example Ge). Finally, OxRAM have a behaviour similar to that of CBRAM in the sense where a conductive filament is formed in an oxide. This filament is formed by accumulation of oxygen vacancies (at least for the family of transition metal based oxides), rather than by accumulation of metal ions. All these memory forms involve the passage of a current through the storage element, which has an electrical resistance varying between a minimum value and a maximum value.
Each of these non-volatile memory technologies has advantages and drawbacks. For example, MRAM are rapid and withstand a virtually unlimited number of reading/writing cycles but have a lower integration density than resistive memories of OxRAM and CBRAM type. OxRAM and CBRAM benefit from low consumption and high integration density, but the variability of the performances between memory points is considerable. These two types of memory moreover have the advantage of being able to be integrated above a logic circuit, for example a microprocessor (“BEOL” integration).
In order to benefit fully from these different advantages, it is customary to combine several memory technologies. Around the microprocessor, it is especially possible to provide MRAM (high speed and endurance) to replace the current SRAM (Static Random Access Memory) of “cache” levels (levels L1 to L3), and PCRAM as the main memory instead of DRAM (Dynamic Random Access Memory). Further away from the microprocessor, OxRAM and CBRAM memories (high integration density and low consumption) advantageously replace the “Flash” memory as mass storage memories.
At the present time, these different non-volatile memories are formed by as many electronic components connected on a same printed circuit (for example a motherboard). However, the integration of two (or more) types of memory within a same component, that is to say on a same semi-conductor substrate, would enable more efficient processing of information by the microprocessor. Indeed, the flow of information would be increased, due to the memories being brought closer together. The co-integration of several types of memory would also facilitate the replacement of volatile memories (SRAM, DRAM) by non-volatile memories and would improve the distribution of the memories around the microprocessor. There thus exists today a need to provide a memory device combining several emerging non-volatile memory technologies on a same substrate in order to benefit simultaneously from the advantages linked to each of these technologies.
Among OxRAM memories, it is possible to distinguish those having a selection device (typically a transistor), which makes it possible during the reading of a memory point to limit leakage currents to adjacent memory points, and those exempt of such a selection device. The latter are qualified as “selector-less”. The article [“Selector-less ReRAM with an excellent non-linearity and reliability by the band-gap engineered multi-layer titanium oxide and triangular shaped AC pulse”; Lee S. et al., Electron Devices Meeting (IEDM), pp. 10.6.1-10.6.4, 2013] describes an example of selector-less OxRAM resistive memory.
Selector-less OxRAM have a more compact structure (by virtue of the absence of selection transistor). The problem of leakage currents therein is resolved by connecting a diode in series with the active layer formed by a metal oxide (e.g. HfO2, Ta2O5). This diode fulfills the function of selector, because the memory then has a non-linear current-voltage characteristic provided with a threshold voltage.
Patent application US2014/0158967 gives another example of selector-less (also called “auto-selective”) OxRAM resistive memory formed by depositing a first layer of metal (for example titanium) on a substrate, by oxidising the first layer of metal in order to form a first oxide layer (TiO2), then by depositing a second oxide layer (e.g. Ta2O5) and a second layer of metal (e.g. Ta). The first and second metal layers form respectively the bottom and top electrodes. The first oxide layer forms a Schottky diode with the bottom electrode and the second oxide layer in (ohmic) contact with the top electrode constitutes the active layer, where the conductive filaments are formed.
To delimit the different memory points, this stack of layers is generally etched, typically by reactive plasma etching. Yet, for dimensions of memory points less than 30 nm, this etching can cause damage to the active material, i.e. the bistable resistance oxide (Ta2O5 in the above example). Moreover, the reactive etching plasma may be the source of numerous structural and/or chemical defects at the edges of the memory points.